Key switches are used to enter information in many electronic products such as computers, telephones, home appliances, toys, cameras, and military equipment. These switches actuate when a key is depressed by a user. The key returns as soon as it is released.
Many mechanically actuated switch types have been invented and are in use, including proximity switches, toggle switches, rotary switches, slide switches, and others. All these switch types have some features that offer some benefits. Key switches offer a set of features--light actuation force, quick action, automatic return, slight feeling of resistance to motion, compactness, ergonomic design, and a motion well-adapted for repetitive finger motions of humans--that is not available as a combination in other switch types. Computers, telephones, and many other electronic products use key switches almost universally for entering alphanumeric characters quickly, conveniently, and repetitively.
A typical key switch includes a key, a pad made of material that conducts electricity, a spring, two electrodes, a support, a housing, and an optional light. Depressing the key moves the pad to a position in which it contacts and forms a bridge between the two electrodes. When this bridge is formed, an electrical path is completed. Completion of the electrical path is the actuation of the key switch. The spring supports the key so that key will not depress unless some outside force is applied. The spring also gives the user a feeling of light resistance when he depresses the key, and returns the key back into its original position ready for the next use as soon as the key is released. To actuate the key switch the user must depress the key with enough force to overcome the spring force. The support is typically a printed circuit board but may be any surface that supports the spring. The housing has apertures for the keys and attaches to the support. A complete system typically includes a sensor in the electronic product that detects when the electrical path has been completed by the formation of a bridge between the electrodes. In some designs, such as an "oilcan" dome or a rubber "button," the key, the pad, and the spring may be the same part. In other designs these elements are separate parts.
In some key switches the depression of the key moves the pad to a position that causes a change in the capacitance or electrical field about a single electrode. In these key switches the pad changes its position relative to an electrode but does not necessarily make contact with that electrode. A complete system typically includes a sensor in the electronic product that senses the change in capacitance or electric field. Alternatively, the pad may be made of a magnetic material. The depression of the key moves the magnetic material to a position that causes a change in the inductance or magnetic field. The complete system typically includes a circuit that senses the change in the inductance or magnetic field.
A "switch mechanism," as used herein, refers to a device that actuates when compressed and returns automatically to its un-compressed position when the compression force is released. A spring means included in the switch mechanism acts to return the switch mechanism to its un-compressed position. Typically, the actuation of the switch mechanism is the completion of an electrical path between two leads of the switch mechanism, but other methods of actuation such as a change in capacitance or inductance are alternatives. Switch mechanisms typically mount on a printed circuit board but may be mounted and wired to any approximately flat surface. Commercially available switch mechanisms are often used as a part of a key switch in order to take advantage of the relatively low cost and good reliability that the manufacturers of the switch mechanisms have gained through their experience and volume of production. Switch mechanisms known as "microswitches" are available commercially from Honeywell, Murata-Erie, and ITT. Another type of switch mechanism known as a membrane switch is available from Bergquist, Tadco, or IEE.
Each key may be an individual part, such as the keys for a common personal desktop computer keyboard. A key cap or a flexible membrane may cover the key itself. Or, a matrix of keys may be molded in one piece out of a flexible material such as the keypads used in many inexpensive telephones. In most cases the key, key cap, flexible membrane, or keypad is identified with letters, numbers, or other mark that indicate the function of the key.
A light source is sometimes used to illuminate the keys. The illuminated keys and flexible membrane or key cap, if present, are made of translucent material. The light source may illuminate the keys from below by shining through the key and covering, if present, so that the location of the the keys and their identifying marks are visible in the dark. The light source may be a light emitting diode (LED), incandescent light, electroluminescent (EL) light, gas discharge light, or other source of illumination. The light source may be placed to shine directly through the keys and flexible membrane or placed so that its light is carried by an optical fiber or reflected by a reflector to shine through the keys and flexible membrane. Some commercial switch mechanisms may include the light source.
Some electronic products, such as handheld radios, cellular phones, ruggedized handheld computers, surveying equipment, navigation equipment including GPS receivers, and similar equipment, are used out-of-doors where they may be rained upon or used in and around swimming pools, rivers, lakes, bays, and oceans. In such environments, the electronic product may be unintentionally or intentionally taken a few feet underwater. These products and the switches they use therefore need to be waterproof to be reliable. Key switches may be designed to be waterproof through the use of a flexible membrane overlaying the switch, use of a diaphragm beneath the key, use of an O-ring around a shaft connecting to a key, use of a keypad compressed between housing parts, and by other methods.
Electronic products intended for deep water applications such as marine salvage, scuba diving, underwater defense and warfare, offshore mineral and oil diving, underwater archeology, and commercial divesuit fishing must not only be waterproof but also must operate at the ambient pressure from sea level to a few hundred feet underwater. Users in these applications will sometimes work from sea level, or above, to two hundred feet underwater in the course of a single day's activity.
Pressure increases by approximately 14.7 pounds per square inch for each 33 feet of depth of sea water or each 35 feet of fresh water. At 200 feet under sea water a key with a surface of 0.5 inches.times.0.5 inches, or 0.25 square inches, has a force of over 20 pounds acting to depress it. In theory, one could design a key switch with a spring force slightly greater than the force that would be generated due to the water pressure at the maximum depth that the product would be used. For example, a product intended for 200 feet but no deeper would be designed with a spring force on each key of 20+ pounds. The problem with this approach is that at sea level, the user would have to push with 20+ pounds in order to operate the keys Even a user with strong fingers finds it inconvenient to push more than a few ounces repeatedly. A product with keys that requires pounds of force would be exceedingly difficult to use over time. Another design approach would be to reduce the force on the keys due to water pressure by reducing the surface area of the keys. The problem with this approach is that the users engaged in deep water applications usually wear thick gloves to protect and keep their hands warm. Smaller keys become more difficult to use when wearing thick gloves.
What is needed is a pressure-compensated key switch with the combination of features of a key switch--light actuation force, quick action, automatic return, slight feeling of resistance to motion, compactness, ergonomic design, and a motion well-adapted for repetitive finger motions of humans--that is waterproof and that operates with light finger pressure from sea level to a few hundred feet below the surface.